In operation of twin belt presses, as exemplified by U.S. Pat. No. 3,951,809, a feed of sludge or slurry having from about 1% solids to about 6% solids by weight is passed into a filter feed box and spread over the width of an essentially horizontally moving first porous filter belt. The sludge layer then passes across a gravity drain zone deck with or without stationary mechanical plows turning or mixing the sludge in the deck zone for initial dewatering, generally to an exit-the-drain zone solids content of about 10%. The resultant cake is then conveyed by the first belt by suitable roller(s) to a drop zone where the resultant partially drained slurry drops off the first belt into a wedge zone between a second (the twin) porous belt converging toward the first belt in proximity to the first belt. The dewatered slurry is sandwiched between and subsequently squeezed between the first and second belts by compressive action of four, five or even more pressure rolls or wedge plates or other pressure devices for further consolidation of the compressible sludge, i.e. removal of further liquor from the cake at various pressurizing (squeezing) stages, until the desired moisture content, i.e. % solids of the cake is obtained. One type of wedge plate includes a single typically perforated spiral-shaped slide surface mounted behind one belt to provide a slow increase in dewatering pressure as a result of tension in the two belts. Slurry, sandwiched between the two belts, travels around perforated and/or non-perforated rolls in a S-roll medium pressure section for further dewatering. After the S-roll section the twin belts travel between a main roll and a series of high pressure rolls. Compression forces are applied by the satellite rolls to the main roll by hydraulic cylinders mounted on the frame to actuate the satellite rolls pressing action. After the last pressure roll the belts separate and the cake, with typical solids content of about 30-45% for municipal biological sludge but dependent on the material and the desired moisture, is discharged from the belts by suitable scrapers. Each additional roller system and each further reduction in % solids content, increases the complexity and expense of the machine. The series of rolls are arranged in staggered configuration with progressively smaller rollers to increase the pressure on the cake and bend it in alternately opposite directions in a serpentine path, shearing the cake. Twin wire filtering machines have a normal cost of from about $50,000 to $400,000 dependent on size, designed throughput, and the particular characteristics of the sludge/slurry being treated.
It has been suggested in the past and practiced in various industries utilizing belt filters to mix dry sawdust or other amendment or absorbent into a feed slurry mixing tank to initially "soak up" or absorb some of the water or liquor in the slurry and improve the rheology of the solids for dewatering. Organic and inorganic coagulants and coagulant aids to tie up slurry fines also have been added to the mixing tank. The resultant mixture was then pumped to the feed slurry distribution box for uniform distribution onto the belt at the beginning of the gravity drain zone and an increment of liquor or water drained from the mixture as the belt moves along the length of the gravity drain zone. It has been found that the resultant cake, after proceeding over the same finite length of the gravity drain zone will be about 6-10% solids in a typical press using typical slurries with or without addition of the sawdust amendment. It is believed that this is so since the free draining of excess water in a gravity drain zone does not include bound water, nor void volume water. These latter are essentially proportional to suspended solids content whether or not amendment solids are present, i.e., sawdust solids will soak up and hold about as much water per unit weight as the original sludge solids will. This extra water associated with the sawdust is counter productive to the dewatering objectives and in general discourages the practice of adding sawdust to a belt press slurry to improve cake rheology, percent capture of fines, and cake solids.